Today food products are often pre-prepared and sold as so-termed convenient meals. For consumer convenience such meals are packed in packages allowing for the heating or cooking of the meal contained therein without removal of the packaging. When heated up, e.g. in a microwave oven, typically moisture contained in the food converts into steam. In order to cope with the rise of the internal gas-pressure such a package comprises a valve, which allows the steam to escape from the package without exploding. At the same time the valve controls the internal steam or gas-pressure as an important parameter for cooking the meal.
Some solutions of these packages rely on a combination of a tray or bowl or the like tightly sealed with a lid. The lid is made of foil that realizes a valve. The valve is created by means of perforations of the foil, wherein the perforations are covered by a layer made of hot melt glue, adhesive, lacquer or the like. Once the hot melt glue is heated—e.g. by the steam created inside the tray—it starts to become permeable for the steam, such that the steam can escape through the perforations with a certain flow rate. This prevents the package from exploding and also allows the pressure to be kept relatively stable within the package while cooking the meal.
Basically this known innovative concept fulfills the requirements for several applications. However, it also shows some drawbacks. For example, as soon as the temperature range used is extended towards extremely low temperatures, the solution may suffer in terms of reliability. At such low temperatures—e.g. in case of shock freezing at −90° C.—the layer covering the perforations tends to become porous or fissured. Hence, it does not any longer tightly seal the perforations.
But also regulatory frameworks may hamper the usability of the package. Typically the food or meal contained in the package is not allowed to come into contact with the material of the layer, which during normal use will not be the case at all. However, improper handling of the package like placing it into a microwave oven up-side-down with the lid facing downward would certainly lead into a situation in which the food via its liquid components would probably come into contact with the melting hot melt glue or layer. Such accidental food contact is heavily undesired.
But also during the manufacturing of the lid and its consecutive handling or transportation several drawbacks are introduced by the layer based solution. Typically the foil would have a thickness of 65 μm while the layer would be in the range of 150 μm. This in fact creates a sort of a bump on the plane foil. The mass production of the lid however requires that several lids must be stored on a roll or in a magazine such that further processing in a highly automated food and package-processing equipment is possible. For both storage systems—roll or magazine—the existence of the bump creates difficulties and may led to malfunction during automated handling of the storage system.
A further back draw of the solution with melting layer is its sensitivity to heat, hence its use is limited to chilled and ambient temperatures.
In other group of solutions label carrying the valve function over a scribed or punctured web is applied. Application of the label can be a process bottle neck either at the converter or on the end user's packaging machine. Offline manufacturing of label alone also imposes high costs on the solution.
Further group of valve solutions use peel able lacquers applied in the sealing area in between the lid and the tray or between two layers of the lidding film in the tray sealing area whilst the food contact layer of the lidding film is cut also in the sealing area and represents a potential risk of leakage of the pack.
Therefore, it is an object of the present invention to overcome the problems identified above and to provide an improved foil of the peel-seal type that realizes a valve of a food package, a package for food that comprises a tray and such a foil or a flexible package for food that is formed by such a folded and sealed together foil, as well as a method of manufacturing such a foil.